// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef NET_DNS_DNS_SESSION_H_
#define NET_DNS_DNS_SESSION_H_

#include <stdint.h>

#include <memory>
#include <vector>

#include "base/lazy_instance.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/metrics/bucket_ranges.h"
#include "base/time/time.h"
#include "net/base/net_export.h"
#include "net/base/network_change_notifier.h"
#include "net/base/rand_callback.h"
#include "net/dns/dns_config_service.h"
#include "net/dns/dns_socket_pool.h"

namespace base {
class BucketRanges;
class SampleVector;
}

namespace net {

class ClientSocketFactory;
class DatagramClientSocket;
class NetLog;
class StreamSocket;

// Session parameters and state shared between DNS transactions.
// Ref-counted so that DnsClient::Request can keep working in absence of
// DnsClient. A DnsSession must be recreated when DnsConfig changes.
class NET_EXPORT_PRIVATE DnsSession
    : NON_EXPORTED_BASE(public base::RefCounted<DnsSession>),
      public NetworkChangeNotifier::ConnectionTypeObserver {
public:
    typedef base::Callback<int()> RandCallback;

    class NET_EXPORT_PRIVATE SocketLease {
    public:
        SocketLease(scoped_refptr<DnsSession> session,
            unsigned server_index,
            std::unique_ptr<DatagramClientSocket> socket);
        ~SocketLease();

        unsigned server_index() const { return server_index_; }

        DatagramClientSocket* socket() { return socket_.get(); }

    private:
        scoped_refptr<DnsSession> session_;
        unsigned server_index_;
        std::unique_ptr<DatagramClientSocket> socket_;

        DISALLOW_COPY_AND_ASSIGN(SocketLease);
    };

    DnsSession(const DnsConfig& config,
        std::unique_ptr<DnsSocketPool> socket_pool,
        const RandIntCallback& rand_int_callback,
        NetLog* net_log);

    const DnsConfig& config() const { return config_; }
    NetLog* net_log() const { return net_log_; }

    // Return the next random query ID.
    uint16_t NextQueryId() const;

    // Return the index of the first configured server to use on first attempt.
    unsigned NextFirstServerIndex();

    // Start with |server_index| and find the index of the next known good server
    // to use on this attempt. Returns |server_index| if this server has no
    // recorded failures, or if there are no other servers that have not failed
    // or have failed longer time ago.
    unsigned NextGoodServerIndex(unsigned server_index);

    // Record that server failed to respond (due to SRV_FAIL or timeout).
    void RecordServerFailure(unsigned server_index);

    // Record that server responded successfully.
    void RecordServerSuccess(unsigned server_index);

    // Record how long it took to receive a response from the server.
    void RecordRTT(unsigned server_index, base::TimeDelta rtt);

    // Record suspected loss of a packet for a specific server.
    void RecordLostPacket(unsigned server_index, int attempt);

    // Record server stats before it is destroyed.
    void RecordServerStats();

    // Return the timeout for the next query. |attempt| counts from 0 and is used
    // for exponential backoff.
    base::TimeDelta NextTimeout(unsigned server_index, int attempt);

    // Allocate a socket, already connected to the server address.
    // When the SocketLease is destroyed, the socket will be freed.
    std::unique_ptr<SocketLease> AllocateSocket(unsigned server_index,
        const NetLog::Source& source);

    // Creates a StreamSocket from the factory for a transaction over TCP. These
    // sockets are not pooled.
    std::unique_ptr<StreamSocket> CreateTCPSocket(unsigned server_index,
        const NetLog::Source& source);

private:
    friend class base::RefCounted<DnsSession>;
    ~DnsSession() override;

    void UpdateTimeouts(NetworkChangeNotifier::ConnectionType type);
    void InitializeServerStats();

    // Release a socket.
    void FreeSocket(unsigned server_index,
        std::unique_ptr<DatagramClientSocket> socket);

    // Return the timeout using the TCP timeout method.
    base::TimeDelta NextTimeoutFromJacobson(unsigned server_index, int attempt);

    // Compute the timeout using the histogram method.
    base::TimeDelta NextTimeoutFromHistogram(unsigned server_index, int attempt);

    // NetworkChangeNotifier::ConnectionTypeObserver:
    void OnConnectionTypeChanged(
        NetworkChangeNotifier::ConnectionType type) override;

    const DnsConfig config_;
    std::unique_ptr<DnsSocketPool> socket_pool_;
    RandCallback rand_callback_;
    NetLog* net_log_;

    // Current index into |config_.nameservers| to begin resolution with.
    int server_index_;

    base::TimeDelta initial_timeout_;
    base::TimeDelta max_timeout_;

    struct ServerStats;

    // Track runtime statistics of each DNS server.
    std::vector<std::unique_ptr<ServerStats>> server_stats_;

    // Buckets shared for all |ServerStats::rtt_histogram|.
    struct RttBuckets : public base::BucketRanges {
        RttBuckets();
    };
    static base::LazyInstance<RttBuckets>::Leaky rtt_buckets_;

    DISALLOW_COPY_AND_ASSIGN(DnsSession);
};

} // namespace net

#endif // NET_DNS_DNS_SESSION_H_
